This invention relates to motorcycles, and in particular to a fork tree by which the fork of a motorcycle is connected to a motorcycle body steering head.
A motorcycle has a frame with a forward wheel, a rear wheel, a motor, a seat and two handle bars connected thereto. The rear wheel is the drive wheel and is connected to the motor, typically by means of a drive chain. The seat is mounted on top of the frame. The front wheel is steerable and connected to the frame by means of a fork, typically hydraulic. The handle bars provide the means for steering manipulation and is connected either directly or indirectly via a fork tree to the fork.
A motorcycle typically is designed so that the front wheel fork is at an angle from a vertical axis through a forward end of the frame. This positioning is generally referred to as a motorcycle's "rake". A motorcycle's rake is generally defined as the inclination angle of the fork from the vertical. See the rake angle "RA" in FIG. 1. Where "V" designates a true vertical and "H" designates a true horizontal. A motorcycle's rake also results in the axle of the forward wheel being positioned forward of said vertical axis. The horizontal distance of the forward wheel's axle from a pre-defined measure is generally referred to as a motorcycle's trail. A motorcycle's trail has been defined as the horizontal distance "T" by which a vertical plane, passing through the front wheel axle "W" trails the point of intersection of the steering axis "A" with a horizontal ground "H".
In order to optimize the riding characteristics of the motorcycle, the rake angle and trail size must be correctly set. If the rake angle and trail are too large, the motorcycle is stable at high speed, but is oversteered and hard to balance at low speeds and in curves. If, on the other hand, the rake angle and trail are too small, or the trail is negative, the motorcycle is easy to steer at low speeds, but has no self steering properties at high speed, and may wobble. Since a rider's size, as well as the desired use of the motorcycle, affect desired rake and trail, some means for adjusting a motorcycle's rake and trail from that set by a manufacturer is desirable.
The precise setting of rake and trail is difficult because of the interrelationship of motorcycle characteristics such as fork tube length and center of gravity. Historically, settings were done by adjusting the spacing between different elements of the steering structure. This is very time consuming and often hit or miss. To provide more precision in setting rake and trail fork tree assemblies were developed. A fork tree assembly initially consisted of a single part connecting the motorcycle's fork tubes to the body steering head. This rapidly evolved into "triple tree" fork assemblies. A triple tree fork assembly typically has an upper tree to which handle bars are attached, a bottom tree to which fork trees are attached, and an intermediate tree element interconnecting the upper and bottom trees and which typically provides means for adjusting the relationship between both thereby affecting the rake and trail of a motorcycle. Triple trees offer rake adjustments allowing the set up of proper rake and trail for desired handling. Raked triple trees can completely alter the low-speed maneuverability, turning radius, and high-speed stability of any bike with a straight frame. A typical prior art triple tree is described in U.S. Pat. No. 4,565,384, "Motorcycle Fork Tree", issued to T. Dehnisch, on Jan. 21, 1986. Another typical triple tree is distributed by Pat Kennedys Custom Cycles, Tombstone, Ariz.
Although triple trees of the prior art provide significant improvements over historical methods for setting rake and trail, it is still a time consuming procedure to adjust rake. Prior art triple trees are also very expensive. Precision repeatability, i.e., the ability to return to and repeat a previous rake and/or trail setting, is also very difficult with prior art triple trees.